LAUSR

Effective compartmentalization of wounds in trees prevents the spreading of decay in wood. Climate extremes including drought can reduce the ability of trees to compartmentalize. In addition, compartmentalization of winter wounds is considered to be weaker than of summer wounds. A combination of drought-stress, winter injury and infection can entirely prevent the compartmentalization and can thus be the major cause for declining tree vitality. For deeper insight into the interaction of climate extremes such as drought and defense responses of trees, the defense reactions of black locust (Robinia pseudoacacia L.) saplings, both well-watered and drought-stressed, to wounding or the pathogen Armillaria mellea were studied. The pathogen was inoculated either in the growth period or during dormancy. All saplings were harvested at the same time, thus, 24 months after inoculation during the growing season and 18 months after inoculation during dormancy, and the extent and effectiveness of compartmentalization were analyzed. Regardless of the season of wounding and infection, the axial spread of discoloration was significantly shorter and the woundwood was almost twice as thick in well-watered compared to drought-stressed saplings. In drought-stressed saplings inoculated in the dormant season, the wounds were not at all sealed and compartmentalized. In these saplings, the presence and spread of the pathogen could be confirmed in the surrounding tissues of the wound. In contrast, the pathogen was not confirmed in tissues of well-watered and drought-stressed saplings inoculated in the active season. To conclude, the well-watered saplings were better able to compartmentalize, and the drought-stressed saplings inoculated in the dormant season, were weakest against wounding and infection and failed to compartmentalize effectively.